1. Field of the Invention
The present invention relates to a process for continuously producing dimethyl carbonate. More particularly, the present invention relates to an industrial process for continuously producing dimethyl carbonate in large scale by a catalytical reaction in gas phase of carbon monoxide with methyl nitrite in the presence of a solid catalyst, while effectively and safely recovering methyl nitrite and nitrogen monoxide from a purge gas withdrawn from a gas-circulating system for this process, and restricting a loss of carbon monoxide.
Dimethyl carbonate is a compound useful as a material for synthesizing aromatic polycarbonates, medicines and agricultural chemicals, and as a solvent.
2. Description of the Related Art
A conventional industrial process for producing dimethyl carbonate by a catalytical reaction in gas phase of carbon monoxide with methyl nitrite in the presence of a solid catalyst comprises, as disclosed in U.S. Pat. No. 5,214,185, a first step of catalytically reacting carbon monoxide with methyl nitrite in gas phase in the presence of a solid catalyst in a reactor to produce dimethyl carbonate; a second step of absorbing dimethyl carbonate produced in the first step by an absorbing medium consisting of dimethyl oxalate in a dimethyl carbonate-absorbing column (absorbing column), to provide a liquid fraction comprising dimethyl carbonate absorbed by dimethyl oxalate and a non-condensed gas fraction containing nitrogen monoxide; a third step of regenerating methyl nitrite by bringing nitrogen monoxide in the non-condensed gas fraction into contact with molecular oxygen and methyl alcohol in a methyl nitrite-regenerating column (regenerating column); and a fourth step of distil-collecting dimethyl carbonate from the liquid fraction produced in the second step and containing dimethyl oxalate in which dimethyl carbonate is absorbed, in an extract-distilling column and a dimethyl carbonate-distilling column, the reaction of carbon monoxide with methyl nitrite and the regeneration of methyl nitrite from nitrogen monoxide are carried out in accordance with the following chemical equations. EQU CO+2CH.sub.3 ONO.fwdarw.CO(OCH.sub.3).sub.2 +2NO EQU 2NO+2CH.sub.3 OH+1/2O.sub.2 .fwdarw.2CH.sub.3 ONO+H.sub.2 O
As the chemical equations clearly show, in the dimethyl carbonate-producing process, methyl nitrite and nitrogen monoxide are not consumed and serve as catalysts in theory. However, in practice, it cannot be avoided that they are partially lost due to the fact that portions thereof are dissolved in the liquid fraction from the second step and in the liquid fraction from the third step, and purged from the gas-circulating system for the process of the present invention through the first, second and third steps. Accordingly, the lost methyl nitrite and nitrogen oxides are compensated by adding methyl nitrite and NOx compound to the gas circulating system.
To compensate methyl nitrite and NOx, a NOx gas is produced by a reaction of sodium nitrite with an inorganic acid, for example, nitric acid or sulfuric acid, and mixed with the circulating gas and a molecular oxygen-containing gas, and the resultant mixed gas is fed into the regenerating column of the third step. This method of producing the NOx gas is simple and advantageous in the production of the NOx gas. However, to produce dimethyl carbonate on a large scale to utilize it for the production of aromatic polycarbonate resins or the like, the above-mentioned method is disadvantageous in that the starting materials are special, and sodium nitrate is produced as a by-product.
In another method of producing the NOx compounds, ammonia is oxidized with air. This method is disadvantageous in that a large amount of nitrogen contained in air accompanies the resultant NOx compounds. Therefore, when the ammonia oxidation product gas is used for the process for producing dimethyl carbonate, namely, the ammonia-oxidation product gas is introduced into the regenerating column of the third step, it is necessary to purge a portion of a circulating gas of the process in a large amount. In this case, methyl nitrite and nitrogen monoxide in the purge gas can be recovered by the method as disclosed in U.S. Pat. No. 4,879,401.
However, this recovering method is disadvantageous in that carbon monoxide contained in the purge gas is difficult to recover and thus the loss of carbon monoxide becomes very large.